Smaller underground passageways for cables and pipes are generally created by first drilling a pilot hole with the use of a drill string. Once both the near and far ends of the pilot hole are open, a reamer is attached to the drill string, at either the near or far end. Then the reamer is forced along the length of the hole while spinning to enlarge it to the required diameter; depending on the size of the pipe or cable to be installed.
Several reamers of gradually increasing sizes may be used, depending on the required final diameter. A number of variations are possible, for instance the reamer can be pushed through the hole (forward reaming) or drawn backwards through (back reaming).
Referring now to FIG. 1 an existing underground reamer 10 is seen to comprise a cylindrical body 12 that in use is connected to the drill string. The cylindrical body has three laterally extending cutting wings 14 that are welded onto the cylindrical body or bolted into recesses 16 in the cylindrical body 12.
While travelling through the hole, for instance in the direction indicated by arrow 18 the reamer is rotated by the drill string in the clockwise direction (anticlockwise from behind as shown by arrow 20). It is the rotation of the wings that enlarges the hole.
The main problem with reamers with wings fixed by welding is they cannot be easily repaired on site. Satisfactory repairs can only be undertaken in a well equipped engineering workshop. This presents another difficulty in that it is extremely difficult to achieve the axial run out tolerances required in a fabrication process. As a later development, reamers with wings bolted into cylindrical body or inner shaft with fixed datums were expected to overcome these problems. However, what occurred with reamers of this kind was that, in use, the bolts 22 were placed under high torsional load and were able to work loose, causing the wings 14 to deflect. This type of deflection increases the diameter of the reamer and makes it susceptible to binding in the hole; which can lead to loss of the reamer and drill string; and sometimes abandonment of the hole. Another problem is that the reamer may not be well stabilised in the hole. This causes vibration which further exacerbates the binding problem.
As a result the wings 14 that are welded or bolted to the cylindrical body 12 often utilize a stabilizing ring 24 which is welded to the tips of the cutting wings 14. Welding the wings to a stabilizing ring has two functions:                To join the cutting wings together to provide lateral stability to the wings.        To smooth out the rotation of the reamer in the hole, reduce the incidence of vibration and provide stability.        
This effectively makes them fixed wings and removes the facility to replace the cutting wings 14 in the field. Consequently, it requires them to be returned to a workshop to carry out repairs.